The question title sums it up fairly quickly. If a character is wielding two shields, one in each hand, does she gain the shield bonus to AC from both, or only the higher one? And where in RAW is this written?

\$\begingroup\$Note that although the AC bonuses don't stack, the shield other properties can stack. For example you can have an interest in wearing a +5 shield and a +1 shield of fire resistance, the full set giving +5 to AC and the fire resistance.\$\endgroup\$
– Anne AunymeNov 28 '16 at 9:43

\$\begingroup\$True... you could also spike the shields, and enchant the shield spikes with non-numeric weapon properties. If you wanted.\$\endgroup\$
– The Amused MuseNov 28 '16 at 19:07

Shield bonuses do not stack with other effects that grant a shield bonus, but nothing says that the bonuses from two shields do not stack. The rules on Armor/Shield Bonuses says

Each type of armor grants an armor bonus to Armor Class, while shields grant a shield bonus to Armor Class. The armor bonus from a suit of armor doesn't stack with other effects or items that grant an armor bonus. Similarly, the shield bonus from a shield doesn't stack with other effects that grant a shield bonus.

(Emphasis mine.) So while a breastplate doesn't stack with a chain shirt, two shields provide bonuses and aren't themselves effects.

\$\begingroup\$Welcome to the site. Take the tour. I edited your answer so that it fits the site's guidelines. (It looks like this answer may've originally been an attempt to comment on The Amused Muse's answer, but that option was unavailable. ) With that in mind, it might be a good idea to review bonuses. Thank you for participating and have fun.\$\endgroup\$
– Hey I Can ChanNov 27 '16 at 15:21

3

\$\begingroup\$This is factual incorrect. -1. Any named bonus does not stack with a bonus of the same name. Shields are no exception.\$\endgroup\$
– SnappieNov 27 '16 at 15:45

\$\begingroup\$If the shield bonus from a shield doesn't stack with other effects that grant a shield bonus, then the effect of the first shield won't stack with the effect of the second shield.\$\endgroup\$
– QuentinNov 27 '16 at 20:50